In modern computer systems, and in particular, personal computers ("PCs"), it is important to ensure that the electric power delivered to electronic components is of the highest quality. "Quality electric power" refers to an electric waveform that is of substantially constant amplitude (or voltage) and wavelength (or frequency) and that is substantially free from other waveform distortions or anomalies, whether periodic (such as harmonic distortion) or aperiodic (such as voltage spikes). If electronic components are not supplied with such high quality power, they may be susceptible to aberrant activity, shortened lifespan or, in extreme cases, outright immediate failure.
In earlier PCs, the electronic components forming the logic circuits therein functioned with a supply of 5 volt power (so-called transistor-transistor logic ("TTL") level power). Most often, power supplies for these computers used solid state switching technology to step down voltage from 110 volt standard household voltage to the required 5 volts. (Power supplies also provided 12 volt power for electrically erasable programmable read-only memory ("EEPROM"), fans, disk drive motors and the like.) These so-called switching power supplies varied the duty cycle of solid state switches coupling the household voltage source to the electronic components. For instance, to step down 110 volts to only 5, the switches was rapidly toggled on and off, coupling the electronic components to the 110 volt source only approximately 5/110ths of the time. This switching produced a crenelated, "square wave" output waveform that, with the help of subsequent filtering, became smoothed and suitable for the electronic components.
The latest generation of PCs employ electronic components for their logic circuitry that require two different voltages of electric power to operate. Some components adhere to the previous 5 volt standard. In the quest to reduce power density and concomitant heat production, newer components (particularly new generations of microprocessor central processing units ("CPUs")) take advantage of newer semiconductor technology and require only 3.3 volts to operate.
Therefore, power supplies have had to be redesigned to supply not only 5 volts, but also 3.3 volts. One way of providing this extra voltage level was to employ a step-down or "buck" voltage regulator to produce the 3.3 volt power. Such regulators typically tap off of the 5 volt power line and, via switching technology, buck the voltage down to 3.3 volts. These regulators functioned well in their steady state, that is, when they were left on. However, in transitions, such as when they were turned on or off, they became susceptible to excessive voltage differences that temporarily existed between their outputs and their inputs.
For instance, one such prior art regulator (to be illustrated and described in greater particularity in the Detailed Description to follow) employed a pulse width modulation ("PWM") circuit, having a noninverted output and an inverted output. The noninverted output was coupled to a control input of a solid state switch coupling an output of the regulator to a 5 volt source. The inverted output was coupled to the control input of a second solid state switch coupling the output of the regulator to ground. The PWM modulation circuit fed synchronous switching signals to the switches to switch alternatively. When one switch was on (for instance, coupling the output of the regulator to the 5 volt source), the other switch was off (so as not to short the 5 volt source to ground, a so-called "shoot-through"), and vice versa.
When the regulator was first switched on, the 5 volt source came on almost immediately, but it took more time for the output voltage to rise to the proper 3.3 volt level (the output had a slower slew rate). This caused a temporary excessive voltage difference to exist between the input and the output, damaging or destroying components within the computer (particularly at-risk was the computer's CPU). Likewise, at power-down, the regulator output voltage dropped more rapidly than the 5 volt source, again causing a harmful excess voltage difference.
In response, regulator designers provided bidirectional diodes bridging the source and the output to limit the amount of voltage difference that is allowed to exist. Unfortunately, the diode protection amounted to a simple limit on the maximum amount of voltage difference that will be tolerated, rather than providing an active control of voltage difference during the transition.
Besides failing to provide an active control of voltage differences, the prior art passive diode "protection" suffered two other notable disadvantages. First, diode forward voltage response curves varied drastically over forward current, allowing the maximum tolerable voltage difference to vary wildly. Second, I.sup.2 T limits inherent in the diodes were sometimes exceeded during conditions of overcurrent or short circuit load, causing the diodes to fail open or shorted, defeating their function and subjecting the sensitive electronic components of the PC to unacceptable power quality.
Accordingly, what is needed in the art is an active control for a voltage regulator that minimizes voltages differences across the regulator during transitions and is not subject to failure modes experienced by passive diodes.